With spread of mobile phones, an RF (Radio Frequency) device primarily intended for a communication interface has attracted attention and it is considered to rapidly evolve in the future.
For example, in case of a mobile phone, when an SOI wafer is used, there is a merit of a reduction in power consumption due to a decrease in leak current. Further, using System on a Chip (SoC) or the like to incorporate the RF device is also considered, and using an SOI wafer can provide a merit of reducing crosstalk between devices in terms of characteristics of an RF transistor.
Here, the crosstalk means propagation of an undesired electrical signal between devices, and the signal is transmitted or received through a capacitor between wiring lines of devices or through a wafer. Although the crosstalk is reduced as a resistivity of the wafer is increased, a wafer resistivity of a device forming portion cannot be actually extremely increased.
However, when the SOI wafer is used, since a buried oxide film layer (a BOX layer) is present between an SOI layer and a base wafer, the crosstalk can be reduced.
Furthermore, since a device is not formed on the base wafer serving as an underlying layer of the BOX layer, a high-resistance wafer can be used without being subject to limitation of device formation, thereby improving RF characteristics.
As described above, at the time of using an SOI wafer for an RF device, utilizing a high-resistance base wafer enables improving RF characteristics. However, when the high-resistance wafer is used, a reversal layer may be generated on an interface between a BOX layer and the base wafer, and an effect of using the high-resistance wafer is reduced in such a case.
As a countermeasure, there is disclosed a technology that increases an interface state density (Dit) to trap carriers in an interface state, thereby preventing characteristics from being degraded due to the reversal layer.
As such a technology, for example, there is known a technology that introduces an intermediate layer such as a polysilicon layer or a nitride oxide to an interface between a BOX layer and a base wafer to prevent the reversal layer from being formed, thereby obtaining an SOI wafer having excellent RF characteristics (see, e.g., Patent application publication No. 2007-507093 and Patent application publication No. 2007-507100).
However, such a technology has a demerit that not only manufacturing steps of an SOI wafer are complicated but also defects on a bonding interface such as voids or blisters are apt to be produced.